drm/i915/dp_mst: Move the joiner dependent code together

drm/i915/dp_mst: Move the joiner dependent code together

Move the calculation of num_joined_pipes and other constraints that
depend on it, into a single block in mst_stream_compute_config().

This groups all joiner-dependent logic together, preparing the code for a
future loop-based evaluation of multiple joiner configurations.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260202103731.357416-8-ankit.k.nautiyal@intel.com